Microwaves are electromagnetic energy waves with very short wavelengths, typically ranging from a millimeter to 30 centimeters peak to peak. In high-speed communications systems, microwaves are used as carrier signals for sending information from point A to point B. Information carried by microwaves is transmitted, received and processed by microwave circuits.
Packaging of RF and microwave microcircuits has traditionally been very expensive. The packaging requirements are extremely demanding—very high electrical isolation and excellent signal integrity through gigahertz frequencies are required. Additionally, IC power densities can be very high. Microwave circuits require high frequency electrical isolation between circuit components and between the circuit itself and the “outside” world (i.e., off the microwave circuit). Traditionally, this isolation was provided by building the circuit on a substrate, placing the circuit inside a metal cavity, and then covering the metal cavity with a metal plate. The metal cavity is typically formed by machining metal plates and connecting multiple plates together with solder or conductive epoxy. The plates can also be cast, which is a cheaper alternative to machined plates. However, one sacrifices accuracy with casting.
One problem attendant with the more traditional method of building microwave circuits is that the method of sealing the metal cover to the cavity uses conductive epoxy. While the epoxy provides a good seal, it comes with a price—high resistance, which increases the loss of resonant cavities and leakage in shielded cavities. Another problem with the traditional method is the fact that significant assembly time is required, thereby increasing manufacture costs.
Another traditional approach to packaging RF/microwave microcircuits has been to attach GaAs or bipolar integrated circuits and passive components to thin film circuits. These circuits are then packaged in the metal cavities discussed above. Direct current feedthrough connectors and RF connectors are then used to connect the module to the outside world.
Another method for fabricating an improved RF microwave circuit is described in U.S. Pat. No. 5,929,728 entitled Imbedded Waveguide Structures for a Microwave Circuit Package, issued on Jul. 27, 1999 to Ron Barnett et al. The '728 patent is incorporated by reference herein for all that it teaches. In general, Barnett teaches a method for fabricating imbedded low-loss waveguide structures in microwave packages via an indented cavity formed in the bottom plane of a metal cover plate. The bottom plane of the cover plate is then fused to a metal base plate. An imbedded shielded cavity is formed when the cover plate and the base plate are joined.
One method for improving RF microwave circuits is to employ a single-layer thick film technology in place of the thin film circuits. While some costs are slightly reduced, the overall costs remain high due to the metallic enclosure and its connectors. Also, dielectric materials typically employed (e.g., pastes or tapes) in this type of configuration are electrically lossy, especially at gigahertz frequencies. The dielectric constant is poorly controlled at both any specific frequency and as a function of frequency. Also, controlling the thickness of the dielectric material often proves difficult.
An improvement upon such methods for fabricating RF microwave circuits is described in U.S. Pat. No. 6,255,730, incorporated herein by reference, entitled Integrated Low Cost Thick Film RF Module naming Lewis R. Dove (co-inventor of the present invention), John F. Casey and Anthony R. Blume as inventors. The '730 patent is assigned to Agilent Technologies, Inc., which is also the assignee of the present invention. The '730 patent describes an integrated low cost thick film RF and microwave microcircuit module. Using an improved thick film dielectric, inexpensive, three-dimensional structures are fabricated on top of a conductive ground plane which is applied to a base substrate. The ground plane forms the bottom electrical shield for the module. A bottom layer of dielectric can be employed to form both microstrip elements and the bottom dielectric for stripline elements. Using an etchable thick film Au process, very small and tightly controlled geometries can be patterned.
Once a shielded RF circuit has been formed, a new challenge opens up, how to introduce signals into the circuit. One option is to use microwave connectors. Microwave connectors provide a very low return loss and low insertion loss and are often used to bring high frequency or high-speed digital signals from the outside world into a microcircuit. However, they are relatively expensive and take up a large amount of space. This becomes a serious problem with circuits requiring many high-frequency connections.
Another possible solution is to attach the center conductor of a semi-rigid coxial line to a microcircuit or circuit board transmission line. However, this exposes the coax line to the edge of a board or substrate, which could couple electromagnetic energy from the coax into the substrate (as a quasi-waveguide mode) rather than to the circuit's transmission line.
Accordingly, the present inventors have recognized a need for method and apparatus to introduce signals into a shielded RF circuit without large interconnects and without coupling electromagnetic energy into the substrate of the RF circuit.